It is widely practiced to attach a label or a sheet printed with product information, such as product number and expiration date, to individual products for product management or for quality guarantee.
Currently available label printing methods include applying liquid ink using a printing plate, thermal transfer using ink ribbon, and inkjet printing. These methods have difficulty, however, in providing a large number of products with individually different information.
Hence, patent documents 1 and 2 (see below) propose a laser-markable laminate comprising a substrate, a base layer, and a hiding layer that absorbs laser light to generate heat and ablates. The colors of the base layer and the hiding layer are combined so that these layers may easily be visually distinguished. When the laminate is imagewise irradiated with a laser beam from the hiding layer side, the irradiated part of the hiding layer is removed to expose the color of the base layer in the form of, for example, letters.
Labels laser-marked with a product serial number and other identifying information are used for certification labeling or approval labeling of machines or automobile parts, and the like. When the label is used in, for example, an engine compartment, the use temperature can elevate to 50° C. or even higher. Labels, such as certification labels, that are attached to any part of the exterior and interior sides of an automobile can be subjected to strong direct sunlight or a severe temperature environment in the tropical or desert regions.
When used under such a severe temperature condition, an ordinary laser-marked label forms a crack or curls, resulting in destruction before the expiration of the usable life of the machine or part.
Using an ordinary label as a laminate for laser marking has been attempted but turned out to be impractical due to poor markability and handling properties.
It is conceivable to use a commonly employed heat resistant resin, such as a polyimide resin or a polyamide resin, to make a pigmented resin layer of a laser-markable laminate. However, most of known heat resistant resins are hard and brittle. For use as a label to be attached to a curved surface of, e.g., a car body, a motorcycle body, or a machine part, such a resin layer has insufficient flexibility, and the label can have its peripheral portion lifted.
The inventors of the present invention proposed in patent document 3 (see below) a laser-markable laminate comprising a pigmented resin layer, a pigmented destructible layer made of a crosslinked acrylic resin and containing a glycol compound, and an adhesive layer. The laminate is attachable to a curved surface and is tamper-proof because of brittleness. The problem with this laminate is that the laser-marked laminate (label) has poor workability in attachment to a substrate due to its softness (lack of moderate stiffness).    Patent document 1: JP 09-123606A    Patent document 2: JP 09-123607A    Patent document 3: JP 2007-021818A